Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_prime_fermat.c

@@ -0,0 +1,41 @@
+#include "tommath_private.h"
+#ifdef MP_PRIME_FERMAT_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* performs one Fermat test.
+ *
+ * If "a" were prime then b**a == b (mod a) since the order of
+ * the multiplicative sub-group would be phi(a) = a-1.  That means
+ * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
+ *
+ * Sets result to 1 if the congruence holds, or zero otherwise.
+ */
+mp_err mp_prime_fermat(const mp_int *a, const mp_int *b, bool *result)
+{
+   mp_int  t;
+   mp_err  err;
+
+   /* ensure b > 1 */
+   if (mp_cmp_d(b, 1uL) != MP_GT) {
+      return MP_VAL;
+   }
+
+   /* init t */
+   if ((err = mp_init(&t)) != MP_OKAY) {
+      return err;
+   }
+
+   /* compute t = b**a mod a */
+   if ((err = mp_exptmod(b, a, a, &t)) != MP_OKAY) {
+      goto LBL_ERR;
+   }
+
+   /* is it equal to b? */
+   *result = mp_cmp(&t, b) == MP_EQ;
+
+LBL_ERR:
+   mp_clear(&t);
+   return err;
+}
+#endif